Thanks for your response! I googled Lomb-Scargle Periodogram and found this MATLAB program:
http://www.mathworks.com/matlabcentral/fileexchange/20004-lomb-lomb-scargle-periodogram
I wrote the following script
x = [0:0.01:2*pi]'; y = [sin(x)]';
[f p Prob] = lomb(x,y,4,1);
[~, index] = max(p)...